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Rapid assessment of soil salinity in tsunami-affected areas. Experiences from Nanggore Aceh Darussalam province, Indonesia The 26 December 2004 tsunami in the Indian Ocean inundated large areas of low lying agricultural land. Soils in these areas are now being used for farming again, but some soils are proving too saline for crop production. Indonesian and Australian scientists working in Nanggore Aceh Darussalam have developed procedures to rapidly assess the salinity of tsunami-affected land and to assess the extent of leaching that has occurred since the tsunami. Fig. 1. Tsunami affected farmland in Lho Nga, Banda Aceh Republik Indonesia BPTP NAD. Indonesian Soils Research Institute 1. Identifying the risk of soil salinity in tsunami-affected areas The level of soil salinity in soils affected by the tsunami varies widely. The type and vigour of plant growth can sometimes be a visual guide to the severity of soil salinity. Plant growth is often patchy in saline soils (Fig.2) and the most salttolerant species may be the only plants that survive (Fig.3). Accumulation of salt crystals on the soil surface and puffy dry soils also indicate salinity. However if the soil has been cultivated there may be no visual indicators of the degree of soil salinity. Fig 2. Poor establishment of crop on saline land in Pante Raja, Pidie. The EM38 instrument shown is used to assess the salinity level of this soil. Fig 3. Salt-tolerant grass in tidal-affected land in Brembang village, Pidie Soil salinity assessment by soil sampling and laboratory analysis Laboratories commonly assess soil salinity by measuring the electrical conductivity (EC) of water extracts of soil samples. EC is commonly expressed in units of deciSiemens per meter (dS/m). The value of soil EC increases with increasing salinity level. Different laboratories may use different ratios of soil to water eg saturated paste (ECe), a 1:2 soil to water ratio, or a 1:5 ratio. Care must be taken when interpreting laboratory data because the different ratios of soil to water will give different laboratory results even though the soil salinity is the same. Soils with an ECe greater than 4 dS/m are classed as saline because the growth of many crops is reduced at this level of salinity. Soil salinity assessment in the field using electromagnetic induction In the field soil EC can be assessed indirectly using electromagnetic induction (EM) methods such as the EM38 instrument shown in Fig.2. The EM38 measures the average EC of the soil in-situ to a depth of approximately 1 m in the field. EM38 measurements increase with increasing soil salinity, clay content and moisture content. They can provide a guide to level of salinity for different texture classes of soils and can be used to guide soil sampling for laboratory analysis. The EM38 instrument can take two types of measurements: vertical, with the instrument sitting vertically on the soil surface (EMv) (Fig. 4), or horizontal with Republik Indonesia BPTP NAD. Indonesian Soils Research Institute the instrument lying on its side (EMh) (Fig. 5). The EMv measurement is more sensitive to soil below 0.45 m than the EMh measurement. The EMh measurement is more sensitive to soil above 0.45 m than the EMv measurement. The two measurements can be compared to indicate how deeply salt may have penetrated the soil. The assessment method can be used to • classify soil salinity risks into low, medium or high risk • evaluate effects of salinity on crop production. • Indicate the extent of salt leaching. • Indicate soil sampling locations Fig.4. EM38 measurement in vertical mode Fig.5. EM38 measurement in horizontal mode Table 1: EM38 ranges for different salinity classes and soil textures Main soil texture class in 0-1 m Sandy soils Loamy soils Clay soils Average EM38 reading (ie EMv+EMh)/2 in dS/m Non-saline* Slightly saline* Saline* <0.4 <0.7 <1.0 0.4-0.7 0.7-1.1 1.0-1.5 >0.7 >1.1 >1.5 *Non-saline is equivalent to average profile ECe< 2 dS/m; Slightly saline is equivalent to average profile ECe between 2 and 4 dS/m, Saline is equivalent to average profile ECe greater than 4 dS/m Republik Indonesia BPTP NAD. Indonesian Soils Research Institute The EM38 has measured EC in farmers’ fields in eastern Nanggore Aceh Darussalam (NAD) province, Indonesia. The measurements have identified several risk factors associated with the most saline land and these are outlined in the table below. Table 2: Salinity risk factors Risk factor Duration of inundation with sea water Soil permeability Inundated by tidal water Number of irrigated rice crops since the tsunami Depth and salinity of shallow watertable Risk of soil salinity affecting crop production LOW MEDIUM HIGH Less than 1/2 day ½ day – 3 days Greater than 3 days Low eg puddled clays with shallow watertable - Moderate eg non puddled loamy soils Tidal water only moderately saline No information for more than 2 rice crops 1-2 No data, but likely to low risk if below 2 m in dry season and EC<2 dS/m. No data, but likely to be medium risk if 1-2 m from surface in dry season and EC 2-4 dS/m. High eg sandy soils irregularly covered by tidal water of high salinity 0 Less than 1 m from surface in dry season. EC > 4 dS/m. Republik Indonesia BPTP NAD. Indonesian Soils Research Institute 2. Estimating the degree of sea water infiltration and salt leaching by comparing the horizontal (EMh) and vertical (EMv) measurements made using the EM38 instrument. Soils normally do not have high salinity levels, so have low EM readings. Normal soils also are often higher in moisture at depth and so often have EMv readings that are greater than EMh. When non-saline soil is flooded by saline water it will have more salinity in the surface horizons than in sub-soil horizons Fig 6a. The inundation will increase EM readings and usually result in EMh being greater than EMv. Leaching of salinity to subsoil depth with non-saline water (eg rainfall) will lower EM readings and eventually result in EMv being greater than EMh (Fig 6b and 6c). A guide to interpreting the relative magnitude of EMv and EMh measurements is given in Table 3 . Fig. 6 Soil salinity profiles and EM readings (a) after tsunami and infiltration of sea water (b) after some leaching, (c) after greater leaching. Republik Indonesia BPTP NAD. Indonesian Soils Research Institute Table 3: Interpreting different combinations of EMh and EMv readings Comparison of EMh and EMv EMh>EMv Indicates soil affected by sea water EMh~=EMv EMh<EMv Average EM38 reading (ie EMv+EMh)/2 Low* Medium* High* Shallow infiltration, Deep infiltration, salt Little infiltration, salt mostly at or closer salt mainly in upper throughout upper 1m 0.3 m of soil. of soil profile with to the soil surface. highest salt Could also indicate concentration near surface. perched wet soil. Indicates soil salt levels are normal (at pre-tsunami level) or approaching normal Normal As above but after some leaching. As above but after some leaching. As above after more leaching. Salinity is highest below 0.3 m. As above after more leaching. Salinity is highest below 0.3 m. • Values of low, medium and high vary with the soil texture profile. Authors Dr Peter Slavich NSW Department of Primary Industries Wollongbar, Australia email: [email protected] Dr Malem McLeod NSW Department of Primary Industries Tamworth, Australia email: [email protected] Dr Natalie Moore NSW Department of Primary Industries Grafton, Australia email: [email protected] T Iskandar BPTP Banda Aceh, Indonesia email: [email protected] Dr Achmad Rachman Indonesian Soils Research Institute Bogor, Indonesia email: [email protected] Date of publication: January 2006 Republik Indonesia BPTP NAD. Indonesian Soils Research Institute